1. Field of the Invention
This invention relates generally to centrifuges, and in certain particular aspects to decanting centrifuges with a rotating bowl, with or without a conveyor or scroll.
2. Description of Related Art
The prior art discloses a variety of decanter centrifuges or “decanters” which, in many embodiments, include a rotating centrifuge bowl rotating at one speed and in which a screw conveyor (“scroll”) revolves at a slightly different speed. Other centrifuges have no such screw conveyor or scroll. Centrifuges are capable of continuously receiving feed in the bowl and of separating the feed into layers of light and heavy phase materials (e.g. liquids and solids) which are discharged separately from the bowl. In those apparatuses with a screw conveyor structure that rotates at a differential speed with respect to the bowl, the conveyor moves or “scrolls” an outer layer of heavy phase or solids slurry material to a discharge port or ports usually located in a tapered or conical end portion of the bowl. Centrifugal force tends to make the light phase material discharge through one or more ports usually located at an opposite end of the bowl. Typically the bowl is solid. Some bowls have port(s) to reject the heavier solids phases.
Centrifugal separation results, preferably, in a discharge containing light phase material with little or no heavy phase material, and heavy phase material containing only a small amount of light phase material. When the light phase material is water and the heavy phase material contains soft solids, it is preferred that fairly dry solids and clean water be separately discharged.
Many different industries use decanter centrifuges in varied applications. They are used in the oil industry to process drilling mud to separate undesired drilling solids from the liquid mud. Some decanter centrifuges, because of their continuous operation, have the advantage of being less susceptible to plugging by solids. Also, they may be shut down for long or short periods of time and then restarted with minimum difficulty, unlike certain centrifuges which require cleaning to remove dried solids. Often the solids/liquid mixture is processed at extraordinarily high feed rates. To accommodate such feed rates, high torques are encountered, much energy is required to process the mixture, and the physical size of the centrifuge can become enormous.
As larger feed volumes are processed in a given centrifuge machine, the clarification capability of the centrifuge decreases due to decreased retention or residence time, partial-acceleration or nonacceleration (slippage) of the feed fluid (the solids/liquid mixture), radial deceleration of the fluid moving through the conveyor, and turbulence created by the movement and/or focusing of large volumes of fluid through ports that tend to transmit and/or focus a high volume flow in an area exterior to the conveyor that induces undesirable turbulence in that area and results in excess wear and abrasion to parts that are impacted by this flow. The turbulent fluid exiting from the ports impedes or prevents solids from flowing to solids exit ports and ports near the centrifuge's drainage deck or “beach” impedes solids flow up the beach.
FIG. 1 shows one typical prior art decanting centrifuge that removes free liquid from separated solids. A rotating bowl creates very high G-forces and forms a liquid pool inside the bowl. The free liquid and finer solids flow towards the larger end of the centrifuge and are removed through effluent overflow weirs. Larger solids settle against the bowl wall, forming a cake. These solids are pushed by a screw conveyor up out of the pool and across a drainage deck (conical section), or “beach”. Dewatering or drying takes place during the process of the solids moving up the beach, with the deliquified solids discharged through a series of underflow solids ports. A gear box connects the conveyor to the bowl, causing the conveyor to rotate in the same direction as the bowl, but at a slightly different speed. This speed differential is required to convey and discharge solids.
The interior end of the feed tube is relatively close to a wall or member defining an end of an acceleration chamber, thus fluid exiting from the feed tube into the acceleration chamber has relatively little space in which to slow down. This relatively high speed fluid is, therefore, turbulent and can wear away parts of the acceleration chamber. Also exiting from the acceleration chamber via exit ports this turbulent-relatively-high-speed fluid can inhibit the desired flow of separated solids both in the bowl toward the solids exit ports and toward the beach area and can wear away parts of the conveyor and bowl adjacent the acceleration chamber exit ports. Rather than dispersing and slowing down the fluid exiting from the acceleration chamber, the exit ports focus and/or speed up the fluid flow.